On-off heater control

ABSTRACT

An On-Off heater control consisting of a switching-type, proportional, temperature regulating control circuit comprising a temperature sensing bridge and differential amplifier which has an output voltage which is a function of input temperature, a triangular voltage function generator, and a comparator and saturating DC amplifier for driving the heating elements.

United States Patent Inventors Carl E. Farmer [56] References Cited k323 IL t UNITED STATES PATENTS ban 0' i; 3,300,622 l/l967 Swain 219/501 Appl No 888,578 3,514,628 5/1970 Pmckaer 219/501 Filed Dec. 29, 1969 Primary Examiner-Bernard A. Gilheany Patented June 22, 1971 AssistanlExaminer-F. E. Bell Assignee The United States of America as A1mmeys-R. S. Sciascia and J. M. St. Amand represented by the Secretary of the Navy gg Q ABSTRACT: A11 On-Ofi heater control consisting of a switching-type, proportional, temperature regulating control U.S. C1 219/497, Circuit comprising a temperature sensing bridge and dif- 219/499,219/501 ferential amplifier which has an output voltage which is a lnt.C1 1105b 1/02 function of input temperature, a triangular voltage function Field of Search 219/499, generator, and a comparator and saturating DC amplifier for 501, 497 driving the heating elements.

r R 1 A N e 1. E 2 VOLTAGE GENERATOR IO 22 MAEHM N 1 DIFF AMP I D.C.AMP 1 21 l CONTROL I TRANSISTOR I 1 a-lf2f Pom HEATER I L J E LE M E N T J PATENTEU m2 2 Ian sum 1 OF 2 TRIANGLE VOLTAGE GENERATOR TRANSISTOR OUTPUT FIG.3

CARL E. FARMER ROBERT W. LONGUEMARE INVENTORS BY A M ATTORNEY ON-OFF HEATER CONTROL The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

There has been a need for a temperature regulator of the proportional-type, but without the disadvantage of high heat dissipation and thus reduced efficiency. Where sensitive circuits are nearby, the usual switching-type of regulator, while having low heat dissipation, is unsatisfactory because rapid switching causes Radio Frequency Interference (RFI The unique characteristics of the present invention are high efficiency, with very little power being dissipated in the control element, combined with a switching arrangement which prevents the generation of Radio Frequency Interference, allowing its use near sensitive receiver circuitry.

Other objects and many of the attendant advantages of this invention will become readily appreciated as the same becomes better understood by reference to the following detailed descriptionwhen considered in connection with the accompanyingdrawings wherein:

FIG. 1 is a block diagram of the heater control circuit,

FIG. 2 shows the differential amplifier circuit portion of FIG. 1,

FIG. 3 is a circuit diagram of the triangle circuit of FIG. 1,

FIG. 4 shows a circuit diagram for the comparator and DC amplifier circuit of FIG. 1,

FIG. 5 illustrates the voltage at various points in the control circuit.

The present invention is comprised of three circuit units as shown in the block diagram of FIG. I, to perform the necessary control functions: A temperature sensing bridge and differential amplifier circuit a triangular voltage function generator 12; and a comparator and saturating DC amplifier an integrated circuit Z differential amplifier whose output is a function of this error signal. R is the feedback resistor for the differential amplifier Z,. Capacitor C l is for frequency compensation of the differential amplifier. Resistors R and R, make up the load for differential amplifier Z with capacitor C used for filtering. Diode CR protects polarized capacitor C from reverse voltage.

The relative values of resistors R,, R R R and R and thermistor R detennine the circuit sensitivity in detecting temperature changes and are selected to provide the required gain for the control loop.

The triangle voltage generator circuit 12 is shown in FIG. 3 and is made up of a Schmitt Trigger consisting of transistors Q and 0,, an integrator transistor 0 and an emitter follower transistor 0,. The control for the Schmitt Trigger is provided by the voltage developed at resistors R, and R The output of the Schmitt Trigger is a square wave voltage. This square wave is integrated and a resulting triangular voltage as shown in FIG. 5B is the output of this circuit.

The comparator and DC amplifier circuit 14 is shown in FIG. 4 and is made up of 2 gain stages, amplifier Z, and control transistor 0,, and emitter followers, transistors 0, and 0,. When amplifier Z is driven positive the output at point 17 furnishes enough base current for transistor O to saturate. Collector current in transistor O is the base current for transistor 0, which also saturates. The collector current of transistor 0,, is the base current for transistor 0-, which also saturates. Therefore the positive input voltage at point 19 saturates transistors'Q Q and 0,. A negative input voltage at 19 will present a back bias for the base-emitter junction of transistor 0 This prevents a base current from flowing in transistor 0,, which prevents a base current from flowing in transistor 0-,. Therefore a negative input voltage at 19 cuts off transistors 0 Q and Q Amplifier Z is used as a high gain voltage comparator. The input at point 19 (noninverting connection) is the sum of the bridge DC amplifier output and the triangular voltage wave output, FIG. 3. Whenever the combined input voltage at point 19 is positive, the voltage at point 17 drives toward +l2 volts. Conversely, when the combined input at point 19 is negative, point 17 drives toward 6 volts. Due to the additional gain of transistors Q and O transistor 0 is correspondingly either full on or full off. Feedback networks formed by resistor R and capacitor C and resistor R and capacitor C slow the switching transition down sufficiently to avoid the generation of Radio Frequency Interference due to fast current transients, and insure loop frequency stability.

Referring to the block diagram of FIG. I, when the thermistor sensor R is colder than the reference resistor R,, the error voltage at point 20 is positive and point 21 is at +l0 volts forexample, such as shown in FIG. 5A. The voltage at point 22, the output of triangle circuit 12, is a constant repetitious triangular voltage of +l volt. The summation of the voltage at points 21 and 22 is as shown in FIG. 5C, for example.

This voltage at point 19, the input of the DC amplifier Z keeps it on and the control transistor is saturated. The current in the heater element resistance R produces heat which in turn is sensed by the sensor thennistor bridge network of FIG. 2.

As the sensor is heated the error signal is reduced until the error is 0 volts at point 2 0 (i.e. the bridge network is balanced). The voltage at point 21, the output of the differential amplifier, is now O.5 v., for example. The summation of voltages at points 21 and 22 will then be +0.5 v. as shown in FIG. 5D, and as shown in FIG. SE at point 25.

The shaded area in FIG. 5D is where the heater R is turned on and the white area is where it is turned off.

When the thermistor sensor R is hotter than the reference resistor R,, the error at point 21 is negative and point 21 is a 5 v. The summation of voltages at points 21 and 22 is as shown in FIG. 5F.

This negative voltage at point 19, the input of the DC amplifier, keeps the control transistor off and zero current flows in the heater element resistance R The actual temperature change required to drive the heater duty cycle from full on to full off depends on the bridge DC amplifier gain which can be made quite high, resulting in very tight temperature control.

The temperature regulator is a proportional controller because the power (not the voltage) supplied to the heater element is proportional to the temperature deviation. The proportional power occurs because the heater voltage is pulsewidth modulated. The time off to the time on is continuously variable causing proportional power control vs. temperature input up to the full output capability of the heating element.

The power dissipation in the control transistor is small because it is operated in a switching mode. Radio Frequency Interference (RFI) is avoided by two precautions: First, the power switching transition is constrained to occur in I millisecond rather than the fractional microsecond range normally used. The transition is still fast enough to maintain high efficiency through low total power dissipation. Second, the pulse repetition frequency is held to a very low value such that significant harmonics fall in the sub audio range. The repetition frequency is made high enough that thermal flicker and control loop instabilities are avoided, however. A Fourier analysis of this trapezoidal waveform shows that the spectral energy distribution is all concentrated at very low frequencies, giving the desired result.

What we claim is:

l. A proportional temperature regulating control circuit,

which prevents the generation of radio frequency inter ference, comprising:

a. a temperature sensing circuit means having an output voltage which is a function of input temperature, said temperature sensing circuit means comprising:

generated in said heating element producing input temperature which is sensed by said temperature sensing circuit means,

. said summed outputs of the temperature sensing circuit 2. the output of said bridge network being connected to 5 means and the voltage function generator having a pulse an integrated circuit differential amplifier whose outrepetition frequency in which significant harmonics fall in put which is a function of the error signal input is a the subaudio range and also avoid thermal flicker suffifunction of the temperature sensed by said thermistor, cient to maintain low power dissipation,

b. a triangle voltage function generator for producing a f. a positive voltage at the summed outputs of said temperarepetitious pulse waveform voltage at its output, said voltl SeflSing Circuit m n n aid voltage function age function generator producing a triangular waveform generator belng fed t ald hlgh gam voltage amplifier lt means driving the amplifier positive and saturating said h ti l t, control transistor means to allow current to flow in said a comparator and switching means having its input conhealer element. a negative voltage at said summed outnected to h summed outputs f id {emperaurc l puts switching said control transistor means to off condisensing circuit means and said voltage function generator, Prevcmmg current from flowing in Said heater said comparator and switching means comprising high heater Vollage pulse wldth modulated gain voltage amplifier whose output is fed to control res umng h Power pp to Sald i element transistor means and feedback network means for slows Propomonal to f tefnpefature devlatlo" sensed ing down switching transition to avoid generation of radio by Sam liemperflmre g clrcu'lt frequency interference due to fast current transients and A dgvlce m cla'm 1 wherem Voltage functlon insure loop frequency stability, the output of said comgenerator comprises a square wave voltage generator whose parator and Switching means connected to Said heating output IS integrated to provideatnangularwave voltage.

element for driving the heating element, and heat 

1. A proportional temperature regulating control circuit which prevents the generation of radio frequency interference, comprising: a. a temperature sensing circuit means having an output voltage which is a function of input temperature, said temperature sensing circuit means comprising:
 1. a resistance bridge network, one leg of which is a thermistor which is exposed to temperature changes from said heating element, the output of said bridge network being an error signal,
 2. the output of said bridge network being connected to an integrated circuit differential amplifier whose output which is a function of the error signal input is a function of the temperature sensed by said thermistor, b. a triangle voltage function generator for producing a repetitious pulse waveform voltage at its output, said voltage function generator producing a triangular waveform voltage, c. a heating element, d. a comparator and switching means having its input connected to the summed outputs of said temperature sensing circuit means and said voltage function generator, said comparator and switching means comprising high gain voltage amplifier whose output is fed to control transistor means, and feedback network means for slowing down switching transition to avoid generation of radio frequency interference due to fast current transients and insure loop frequency stability, the output of said comparator and switching means connected to said heating element for driving the heating element, and heat generated in said heating element producing input temperature which is sensed by said temperature sensing circuit means, e. said summed outputs of the temperature sensing circuit means and the voltage function generator having a pulse repetition frequency in which significant harmonics fall in the subaudio range and also avoid thermal flicker sufficient to maintain low power dissipation, f. a positive voltage at the summed outputs of said temperature sensing circuit means and said voltage function generator being fed to said high gain voltage amplifier means driving the amplifier positive and saturating said control transistor means to allow current to flow in said heater element, a negative voltage at said summed outputs switching said control transistor means to off condition preventing current from flowing in said heater element, the heater voltage being pulsewidth modulated resulting in the power supplied to said heater element being proportional to the tEmperature deviation sensed by said temperature sensing circuit means.
 2. A device as in claim 1 wherein said voltage function generator comprises a square wave voltage generator whose output is integrated to provide a triangular wave voltage.
 2. the output of said bridge network being connected to an integrated circuit differential amplifier whose output which is a function of the error signal input is a function of the temperature sensed by said thermistor, b. a triangle voltage function generator for producing a repetitious pulse waveform voltage at its output, said voltage function generator producing a triangular waveform voltage, c. a heating element, d. a comparator and switching means having its input connected to the summed outputs of said temperature sensing circuit means and said voltage function generator, said comparator and switching means comprising high gain voltage amplifier whose output is fed to control transistor means, and feedback network means for slowing down switching transition to avoid generation of radio frequency interference due to fast current transients and insure loop frequency stability, the output of said comparator and switching means connected to said heating element for driving the heating element, and heat generated in said heating element producing input temperature which is sensed by said temperature sensing circuit means, e. said summed outputs of the temperature sensing circuit means and the voltage function generator having a pulse repetition frequency in which significant harmonics fall in the subaudio range and also avoid thermal flicker sufficient to maintain low power dissipation, f. a positive voltage at the summed outputs of said temperature sensing circuit means and said voltage function generator being fed to said high gain voltage amplifier means driving the amplifier positive and saturating said control transistor means to allow current to flow in said heater element, a negative voltage at said summed outputs switching said control transistor means to off condition preventing current from flowing in said heater element, the heater voltage being pulsewidth modulated resulting in the power supplied to said heater element being proportional to the tEmperature deviation sensed by said temperature sensing circuit means. 